(a) Field of the Invention
The present invention relates to a lens system for single-lens reflex cameras, more particularly to an attachment lens system for single-lens reflex cameras so interposed between an ordinary interchangeable lens system and camera body that TTL auto-focus mechanism is able to be added to a single-lens reflex camera.
(b) Description of the Prior Art
Conventionally, as an auto-focus device for single-lens reflex cameras, a method in which the distance measurement is effected, using a so-called triangulation, by a distance measuring optical system different from a photographing lens system attached on the camera body and the focusing is effected by transmitting the distance information obtained thereby to the focus driver of photographing lens system is well known. However, in this method, because of the existence of parallax between the photographing lens system and distance measuring optical system, while an accurate focusing is able to be effected on a comparatively far object, it is easy to happen the distance measuring errors on a near object owing to the increase of the influence of parallax. And, as it is necessary that the base length for the distance measurement is to be long in some degree, there is a defect that the distance measuring optical system or lens system as a whole will be large. Further, as it is necessary that the photographing lens system and distance measuring optical system are interlocked as a set, it has a fatal defect that the auto-focus function is not obtainable in case a special interchangeable lens system provided with the distance measuring optical system for autofocus is not used.